1. Field of the Invention
The present invention relates to a logging device for detecting and mapping vertical conductive fractures in areas of the earth surrounding a bore hole.
Techniques are being investigated for extracting energy from hot dry rock. The present approach is to drill a hole into a hot solid rock formation, hydrualically fracture the rock, determine the fracture orientation, intersect the fracture with a second drill hole, and circulate a working fluid, such as water, between the two drill holes through the vertical fractures to extract thermal energy. At present there is no proven method of mapping vertical fractures from a single borehole to enable directional drilling of the second hole with sufficient accuracy to intersect the fracture. The present invention comprises an instrument capable of mapping vertical conductive fractures in an electrically resistive bedrock. The fracture is mapped using magnetically induced eddy currents in the water filled fracture, employing an instrument somewhat similar to the induction sondes conventionally employed by the oil industry.
2. Description of Prior Art
In the present dry hot rock energy extraction techniques, difficulty has been encountered in establishing a low impedance liquid connection between the two bore holes. The fractures connecting the two bore holes appear to be roughly circular vertical discs of several hundred meters in diameter and several millimeters wide. These fractures are at a depth of about 3000 meters. The water circulating between the two bore holes reaches a temperature of 275.degree. C. at a pressure of 10,000 psi. This water has a resistivity of approximately 10 ohm-m at 250.degree. C. If the fracture width is 3 millimeters, the fracture corresponds to an equivalent sheet resistivity of 3000 ohm per square. Resistivity of the surrounding rock is considerably greater than 3000 ohm-m. While the petroleum industry employs several techniques to attempt to map fracture systems, these generally do not lend themselves to the type of fracture system that is to be measured in connection with hot dry rock geothermal systems.
The present invention enables fracture mapping employing a single bore hole and at any depth. Alternating magnetic fields are set up around a conducting medium in the fracture that generate eddy currents therein. These eddy currents in turn generate magnetic fields that are sensed by a receiver, enabling mapping into the rock formation.
The vertical conductive fracture detector senses eddy currents induced in the fluid in the fracture by a transmitting coil contained in the instrument. The eddy currents can be detected by measuring their magnetic field with a receiving coil. The fracture orientation is found by rotating the coils and finding the angle .beta. for which the signal is maximum. This mapping involves several elements. The orientation of the instrument case must be determined by means of a reference. The coils are rotated to determine the greatest received signal. This rotation can be accomplished electronically, involving no mechanical motion, if two identical coil sets orientated at right angles to each other are used. The coils are located in a nonconductive housing. However, the conductive bore hole fluid must be displaced in the vicinity of the coils.